You should use both, virtual servers page and the application gateways page.

In the virtual servers:
set LAN IP of your Mac where you will use iChat
for type use TCP or UDP as defined for each specific port in the Notes section of the Apple support page (UDP for 5060, 5190, 5223, 5678, TCP for 5220, 5222, 5223, 5190)
use the same port number (as above) for LAN and Public
check Enable
click Set

In the Application Gateways page:
set trigger port to 16384
set Type to UDP
set Public Port to 16384-16403
Public Type to UDP
check Enabled

If this configuration does not work as expected , try turning on the UPnP setting on the router. It can be found under System Settings menu.